Two ideas collide once you reach your seventies, and the collision costs people real money every year.
Idea one: Roth conversions are good, so I should keep converting.
Idea two: I have to take a required distribution, so I will just convert that instead of spending it.
The second idea is not allowed. It is one of the cleanest prohibitions in the retirement code, and the correction process for getting it wrong is genuinely unpleasant.
Here is how the two interact, what order everything has to happen in, and why conversions are still worth doing after the requirements begin.